This invention relates to a dual circuit vehicle hydraulic brake system and more particularly to a tandem master cylinder for such a system.
German Patent DE-OS 2,604,663 discloses a master cylinder for a vehicle hydraulic brake system which is connected with the front-wheel and rear-wheel brakes through brake piping and includes, in a stepped bore of a housing, two telescoping pressure pistons slidable in the housing bore. Each pressure piston forms with an annular surface a boundary for a pressure chamber in which there is disposed a port for a fluid conduit leading to the front-wheel brakes. A first of the pressure pistons has a multiple stepped circumferential surface, with the ends of the piston being of a smaller diameter, one end being guided in the smaller step of the housing bore and the other end being guided in a coaxial bore in the second pressure piston. The first pressure piston includes a coaxial bore in which two stepped pistons are located close to the piston ends, each stepped piston being loaded by an associated control spring. With their smaller steps, the stepped pistons form a boundary for a regulator input which is in communication with the above-noted pressure chamber, and with their larger steps, they form a boundary for a regulator output which, where applicable, is connected with a fluid port for the rear-wheel brakes through suitable fluid passageways.
Each stepped piston is assigned a valve, and the closure members of these valves are secured to the ends of an operating piston. The operating piston is disposed in the coaxial bore of the first pressure piston between the two stepped pistons and is subjected to the unregulated pressure of the one brake circuit on its one end and to the unregulated pressure of the other brake circuit on its other end. When the brake is applied, closing of the valves will occur above a predetermined pressure, and a reduced brake pressure will be supplied to the rear-wheel brakes, while the front-wheel brakes will be subjected to the unreduced pressure.
If one brake circuit fails, the differential of pressure acting on the operating piston will cause actuation of the operating piston, thereby preventing closing of the valve in the intact brake circuit so that there occurs no reduction of the brake pressure for the rear-wheel brake.
The arrangement known from the above cited German Patent is costly from the point of view of machining and assembly because it necessitates a large number of individual components which are sometimes of complicated construction. From this results high manufacturing costs. Moreover, such a design is only realizable with master cylinders having a relatively large diameter, because the brake pressure for the front-wheel brakes can only be produced in the annular chambers bounded by the pressure pistons and not on a cross-sectional area corresponding to the piston diameter.
In the event of failure of a brake circuit, the lost travel of the brake pedal is increased considerably because pressure is not built up in a pressure chamber. This increased pedal travel is increased still further as a result of the displacement of the operating piston and the high brake fluid consumption caused thereby. In the event of failure of a brake circuit, in which case it is important to achieve the best possible braking effect with the intact brake circuit, pressure can be built up in the brake system only after the sum of the additional lost pedal travels has been overcome.
The operating piston is only actuated in the event of a defect of the brake system, the ring seals sealing the operating piston are therefore not normally moved. It is, however, known that seals which are not moved over a prolonged period of time are destroyed when suddenly loaded. It is then necessary to repair the master cylinder.